DE19640663C2 - Solid lubricant layer, method of manufacture and its use - Google Patents

Description

The invention relates to a solid lubricant layer based on at least one compound of the formula MeX _n , wherein
Me molybdenum, tungsten, niobium, tantalum or titanium,
X is sulfur, selenium or tellurium,
with a layer structure of main layers made of the compound of the formula MeX _n with a layer thickness of 5 nm to 1000 nm, which are separated by thinner intermediate layers from another material, and a method for producing such layers.

If molybdenum disulfide is applied to a substrate by PVD, only in the Initial stage of growth with a layer of molybdenum disulfide crystallographic (0001) texture (basal orientation), i.e. one to the coated one Parallel arrangement of the layers of the molybdenum disulfide Crystal lattice. Already after reaching a very small layer thickness of about 0.1 µm the basal orientation no longer dominates, d. H. the average angle between the (0001) planes of the molybdenum disulfide crystallites and the substrate surface enlarges more and more until it forms a further layer growth Columnar or columnar structure comes in which the (0001) planes of the crystal lattice run almost perpendicular to the substrate surface.

The short lifespan of such a molybdenum disulfide layer applied by PVD is due to the columns breaking off within the columnar structure when subjected to mechanical stress. The lubricating effect that can be achieved is exclusively due to the thin molybdenum disulfide layer with a basal orientation. In addition, crystallographic textures other than the basal orientation promote the diffusion of oxygen and water into the molybdenum disulfide layer, which leads to the formation of molybdenum oxide (e.g. MoO ₃ ), which causes embrittlement of the smear layer. This can be attributed to the fact that only the (0001) planes of the crystal lattice are chemically inert; A chemically inert layer surface can only be created using a (0001) texture (basal orientation). This corresponds to a molybdenum disulfide layer with a basal orientation (0001 levels lie parallel to the layer surface).

US Pat. No. 5,282,985 discloses a multilayer solid lubricant layer with stoichiometric MeX ₂ as the main layer and intervening intermediate layers which, with a suitable choice of the layer thicknesses, eliminates the problem of the breaking columns. The compounds described there have good lubricant properties due to their layered structure. This applies in particular to layers with molybdenum disulfide (MoS ₂ ). However, the service life of such coatings is still far below expectations.

DE 35 16 933 C3 discloses a method for producing thicker laminar MoS ₂ layers, ie with a basal or 0001 orientation. For this purpose, however, the water vapor partial pressure in the sputtering chamber is reduced to an extremely low value, so that this process is associated with a correspondingly high outlay.

From JP 61-231093 A it is known to molybdenum disulfide layers by epitaxial Growing up with a molecular beam, i.e. by growing up in individual molecular ones To create layers. Semiconductor wafers are known to be made by such processes manufactured. For this, substrate carriers with extreme smoothness are required. Solid lubricating layers on tools cannot be produced in this way become.

When molybdenum disulfide is coated by physical vapor coating, especially plasma PVD, which is applied to a substrate, also occurs in the known ones Multi-layer systems have a solid lubricant layer with regard to their service life however still falls far short of expectations. In addition, one rapid embrittlement of molybdenum disulfide smear layers produced by PVD ascertain.  

The object of the invention is to provide a solid lubricant layer of the type mentioned to provide, which is characterized by a long life and not for Embrittlement tends.

According to the invention, this is characterized by that in claim 1 Solid lubricant layer reached. In claims 2 to 6 are advantageous Embodiments of the solid lubricant layer according to the invention specified. in the Claim 7 is a preferred method for producing the invention Solid lubricant layer, which is characterized by the measures of the claim 8 is further trained. The claim 9 relates to preferred uses of the solid lubricant layer according to the invention.

The invention is therefore based on the knowledge that the proportion of sulfur in the molybdenum disulfide or in general in X in MeX _n influences the layer properties. If the proportion is in the sub-stoichiometric range, the properties of the solid lubricant layer deteriorate. Therefore, the proportion of sulfur or X according to the invention is in an over-stoichiometric ratio compared to the metal Me.

In addition, the structural integrity of the MeX _n main layers of the solid lubricating layer according to the invention is improved by introducing a metal or a metal alloy with a content of up to 20% by volume, preferably up to a maximum of 5% by volume. The metal or metal alloy dispersed in the MeX ₂ main layer makes the main layer more compact and thus obtains an improved structural integrity.

Acoustic microscopy showed that a molybdenum disulfide Layer in which 5% chromium is dispersed, has a modulus of elasticity of about 20 GPa, while molybdenum disulfide has a modulus of elasticity of only 1 GPa at most. Hereby the lifespan of the solid lubricant layer is therefore multiplied.  

In addition, the brittleness of the MoS ₂ layer is significantly reduced by the excess of stoichiometric sulfur.

Fig. 1 shows two schematic representations, namely

Fig. 1a a molybdenum disulfide layer with a columnar structure and

Fig. 1b a multilayer molybdenum disulfide solid lubricating layer according to the invention with overstoichiometric sulfide content.

As can be seen in Fig. 1a, when molybdenum disulfide is applied by PVD to a substrate 1 in the initial stage, a MoS ₂ layer 2 with a basal orientation is formed ( 0001 ). Even when a very thin layer thickness of layer 2 is reached, the basal orientation is no longer dominant. That is, the average angle between the (0001) planes of the molybdenum disulfide crystallites and the surface of the substrate 1 increases more and more until a columnar or columnar structure 3 is formed as the layer grows further. This can be prevented by interrupting the MoS ₂ layers several times by means of intermediate layers. However, such coating systems also fall short of expectations in terms of their service life and embrittlement.

According to Fig. 1b, the solid lubricant layer according to the invention has a multilayer structure consisting of main layers 2 made of overstoichiometric molybdenum disulfide with a basal orientation, or generally of the compound of the formula MeX _n with 2 <n <4, the main layers being separated by intermediate layers 4 made of another material , and a metal or a metal alloy with a content of up to 20 vol.% is dispersed in the main MeX _n layer.

Fig. 2 shows a SEM image of a solid lubricant layer according to the invention with a total thickness of approx. 2.5 µm on a substrate, the superstoichiometric molybdenum disulfide main layers each having a layer thickness of approximately 80 nm and the chrome metal intermediate layers each having a layer thickness of approx Have 7 nm. Chromium was also used as the dispersed metal in the main layers.

For physical deposition from the gas phase (PVD) according to the invention especially sputtering, i.e. plasma PVD applied, and above all that Magnetron PVD or sputtering. A plate-shaped coating source or Target (cathode) used, which have been produced for example by powder metallurgy is.

If the substrate, as usual, with its surface the plate-shaped target opposite, that is, arranged parallel to the target, it has been shown that the applied molybdenum disulfide layer on the substrate is not satisfactory Has lubricating properties. As has been shown, this is due to a substoichiometric Mo: S ratio less than 2 attributable.

The molybdenum disulfide layers according to the invention with an overstoichiometric Sulfur content, however, show a particularly pronounced lubricating effect. To a Obtaining an over-stoichiometric ratio is in the process according to the invention not to be coated substrate surface parallel to the plate-shaped target, but arranged obliquely, preferably at an angle of about 60 ° to 90 °. To this A molybdenum disulfide target can be used in a simple manner Obtained molybdenum disulfide layer with a stoichiometric excess of sulfur become. Which is why when the surface to be coated is arranged obliquely plate-shaped molybdenum sulfide target an over-stoichiometric sulfur content in the evaporated layer is on different sputtering rates for sulfur and Molybdenum and the high volatility of the sulfur in the coating chamber prevailing pressure / temperature conditions.

The intermediate layers between the MeX _n main layers are also applied using PVD, in particular magnetron sputtering, using a further target made of the intermediate layer material.

The intermediate layers should be as thin as possible to ensure the lubricating properties to impair the solid lubricant layer as little as possible. The is preferably Layer thickness of the intermediate layers in each case 1 nm to 100 nm, maximum 300 nm Intermediate layers are in any case thinner than the main layers, preferably around at least an order of magnitude thinner. If the intermediate layers are too thick, the Lubrication effect impaired.

By introducing the thin intermediate layers, the growth of the Molybdenum disulfide layer repeatedly interrupted. That is, before it becomes a columnar formation of the molybdenum disulfide layer comes, the PVD process interrupted and an intermediate layer applied by PVD. In the further PVD process with molybdenum disulphide, this grows up again initially with a basal orientation. To Reaching the specified molybdenum disulfide layer thickness, i.e. before insertion columnar growth, the PVD process is interrupted again. This episode of alternating application of a molybdenum disulfide main layer and one The intermediate layer is repeated until the desired total thickness of the Solid lubricant layer is reached, which are for example between 10 microns and 100 microns can.

The layer thicknesses of the individual main layers and the individual intermediate layers can be different to ensure that lubricant is dispensed as required guarantee. The lubricating effect of the solid lubricating layers should always be high his. For example, there is a higher need for lubrication in the running-in phase. By Modification of the layer system structure can take the higher lubrication requirement into account be worn. The stress profile is a selection criterion for the multilayer To consider layer structure. A gradient structure can be provided, wherein the layer thickness increases from the substrate to the outside, or a periodic or Non-periodic layer structure is available. Instead of or in addition to the gradient build-up a variable chemical composition can also be provided.

Since a dominance of the solid lubricant layer according to the invention Basal orientation of the molybdenum disulfide layers is guaranteed  Solid lubricating layer according to the invention also chemically inert, d. H. it is not Observe embrittlement in the air or through water. According to the invention compact, non-porous solid lubricant layer achieved. The coefficient of friction of the Solid lubricant layers are always less than without a solid lubricant layer (not only with abrasive wear). Because the solid lubricant layer contributes to the generation of heat Counteracts sliding / rolling, forming, machining and general abrasive loads, it leads to a longer service life of components and tools.

A substantial improvement in the structural integrity of the MeX _n main layers of the solid lubricating layer according to the invention is achieved by introducing a metal or a metal alloy with a content of up to 20% by volume, preferably up to a maximum of 5% by volume. Due to the metal, metal alloy or metal compound dispersed in the MeX _n main layer, the main layer becomes more compact and thus receives an improved structural integrity.

The metal or metal alloy in the main layers is also dispersed by PVD in the MeX _n main layer. For this purpose, for example, a MeX _n target can be used, from which a piece has been removed and replaced by a corresponding piece made of the metal, the metal alloy or the metal compound. The metal or the metal alloy can also be introduced into the MeX _n target by powder metallurgy. The use of several coating sources (cathodes) offers another possibility.

The intermediate layer also preferably consists of a metal, a Metal alloy or a metal compound.

The same metal or the same metal alloy or metal compound can be used both for dispersing in the MeX _n main layer and as an intermediate layer or different metals, metal alloys or metal compounds for dispersing in the MeX _n main layer or as an intermediate layer.

The metal or the metal alloy or the metal compound can be a metal of
Group Ib, in particular copper, silver or gold,
Group IVa, especially germanium, tin or lead,
from group IVb, in particular titanium, zirconium or hafnium,
the group Vb, especially tantalum,
Group VIb, in particular chromium, molybdenum or tungsten, and
Group VIIIb of the periodic table, in particular iron, cobalt, nickel, palladium, platinum or iridium, or a lanthanoid, such as cerium or samarium, or magnesium, manganese, rhenium or scandium.

In particular, chromium metal has proven to be suitable, as well as chromium alloys, such as Nickel chrome alloys.

The sulfides, selenides, telorides and borides have been found in particular as metal compounds and carbides of these Group Ib, IVa, IVb, Vb, VIb and VIIIb metals of the Periodic Table proven suitable.

However, non-metallic materials can also be used, in particular for the intermediate layers are used, in particular polymers such as polytetrafluoroethylene.

As tribological experiments have shown, a molybdenum disulfide layer according to the invention with a multilayer structure according to FIG. 2 with chrome metal intermediate layers and a chromium content of 4% by volume in the main layers has a service life which is more than three times as long as a single-layer columnar molybdenum disulfide layer.

As substrates on which the solid lubricating layer by PVD according to the invention applied, come in particular substrates made of metal, especially steel, as well as ceramic or plastic in question.

The solid lubricant layer according to the invention is practically abrasive for everyone Suitable surfaces. It is particularly suitable for rolling and plain bearings, for Forming tools and cutting tools. With forming tools  provide the surfaces in particular with the solid lubricant layer according to the invention, which are exposed to high sliding friction. With cutting tools in particular the area of the tool with the invention Provide a solid lubricant layer over which the chip is led away from the cutting edge.

In plain bearing applications, lifetime lubrication with the solid lubricant layer according to the invention, and the fact that higher loads completely new opportunities can arise.

The lubricating layer according to the invention also leads to a substantial reduction in the Frictional heat development in machining processes such as drilling or forming processes. As a result, coolants and lubricants can become superfluous and thus the Disposal costs for these funds are eliminated.

In addition, the solid lubricant layer according to the invention is for transmission parts usable, for example gears, switches, pumps and the like.

Use of the solid lubricant layers in the medical field is also conceivable. At Proof of biocompatibility could be, for example, the one according to the invention Smear layer can be applied to parts of protes.

Claims (9)

1. solid lubricant layer based on at least one compound of the formula MeX _n ,
wherein
Me molybdenum, tungsten, niobium or tantalum, and
X is sulfur, selenium or tellurium,
with a layer structure of main layers made of the compound of the formula MeX _n with a layer thickness of 5 nm to 1000 nm, which are separated by thinner intermediate layers made of another material, characterized in that 2 <n <4 and in the MeX _n - Main layers ( 2 ) a metal or a metal alloy with a content of up to 20 vol .-% is dispersed. 2. Solid lubricant layer according to claim 1, characterized in that the intermediate layer ( 4 ) consists of a metal, a metal alloy, a metal compound or a polymer. 3. Solid lubricating layer according to claim 1 or 2, characterized in that the metal or metals of the metal alloy from which the dispersion or the intermediate layers ( 4 ) consist, or the metal of the metal compound from which the intermediate layer ( 4 ) consists, a metal the group Ib, IVa, IVb, Vb, VIb or VIIIb of the periodic table or a lanthanoid or magnesium, manganese, rhenium or scandium. 4. solid lubricant layer according to one of claims 1 to 3, characterized in that the metal compound is a sulfide, selenide, telluride, boride or carbide. 5. solid lubricant layer according to claim 2, characterized in that the polymer Is polytetrafluoroethylene. 6. Solid lubricant layer according to one of the preceding claims, characterized in that the thickness of the main layers ( 2 ) from the substrate ( 1 ) increases to the outside or corresponds to a periodic or non-periodic layer structure. 7. The method for producing the solid lubricating layer according to one of the preceding claims, characterized in that the MeX _n main layers ( 2 ) and the intermediate layers ( 4 ) are applied by physical vapor coating (PVD) to the surface of the substrate to be coated, the to coating surface of the substrate ( 1 ) is arranged obliquely to the plate-shaped PVD coating source. 8. The method according to claim 7, characterized in that the physical Steam coating by Magnetron PVD. 9. Use of the solid lubricant layer according to one of claims 1 to 6 for rolling or plain bearings, forming tools, cutting tools and gear parts.